Display device

ABSTRACT

The invention provides a display device ( 26 ) including a plurality of discrete display segments ( 2 ). Each display segment ( 2 ) is provided with a drive circuit for driving the display elements arranged within the display area. The display device may be provided either as a passive, active or direct pixel addressed array. By interconnecting a number of display segments, a large area display can be achieved without the requirement for long electrodes. This reduces the electrical resistance and parasitic capacitance of the addressing electrodes, enabling the display to provide improved luminance in a displayed image and to operate at higher speeds, providing improved resolution. An active matrix addressing scheme can also be implemented using relatively low mobility organic thin film transistors.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Division of application Ser. No. 11/141,005 filed Jun. 1,2005, which in turn is a Division of application Ser. No. 10/276,597filed Nov. 19, 2002, which is a National Stage of Application No.PCT/GB02/00412, filed Jan. 30, 2002. The entire disclosures of the priorapplications are hereby incorporated by reference herein in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to display devices and, in particular, todriver arrangements for display devices.

2. Description of Related Art

A variety of display devices, such as liquid crystal displays or lightemitting diode (LED) displays, are in widespread use. Recently, afurther type of LED display has been proposed in the form of anaddressable electroluminescent display. The electroluminescent displaydevice comprises a mix of organic materials such as organic polymers orsmall molecules sandwiched between an anode and a cathode supported by asolid substrate, such as, for example, a glass, plastics or siliconsubstrate, the organic materials providing the light emitting elementsof the display.

Organic material LED's have a much sharper response characteristic thanliquid crystal display devices. The organic LED devices have very sharp‘turn on’ and ‘turn-off’ characteristics in response to an appliedcurrent which provides such displays with improved contrast incomparison to liquid crystal displays. In addition to improved contrast,the organic materials are also considered to provide significant benefitin terms of fabrication.

For organic LED displays incorporating organic polymer materials as thelight emitting pixels, the organic polymer materials may be deposited onthe substrate using fabrication techniques which cannot be adopted tomanufacture liquid crystal or conventional light emitting diodedisplays. One method which has been proposed is to deposit the organicpolymer materials onto the substrate using inkjet printing in which thepolymer materials are deposited as discrete drops of the material ontopredisposed deposition sites provided as a matrix configuration on thesubstrate. The use of inkjet printing can be particularly advantageousfor colour displays because the various organic polymer materials whichcomprise the red, green and blue LED's at each pixel of the display canbe deposited in the required predefined patterns without the need forany etch process steps. In the case of small molecule type organic LEDdisplays, shadow mask evaporation is generally applied to form thecolour pixels.

Furthermore, as the active materials of the display are organic polymermaterials, they may be deposited onto any suitable substrate material,including plastics materials in the form of a continuous, flexible andspoolable sheet. The characteristics of the organic polymer materialslend themselves, therefore, to the fabrication of very large areamonochrome or colour display devices containing very large numbers ofrows and columns of the pixels of active material making up the displayarea of the display device.

An organic electroluminescent display may be driven using either anactive or a passive matrix addressing system. The display elements whichcreate the light output at any pixel of the display are, in essence,provided by organic light emitting diodes. These are current drivendevices, so when an active matrix addressing scheme is used to addressthe display to create a displayed image, two transistors per pixel ofthe displayed image are provided, the first to act as a switch so as toallow a data signal to be passed to a second transistor which acts as acurrent driver for the LED of the pixel, thereby to determine thebrightness for the pixel.

A passive matrix addressing scheme is shown schematically in FIG. 1. Thedisplay element 2 shown in FIG. 1 comprises a substrate 4 supporting anarray of strip-shape electrodes 6 which constitute the anode electrodesof the display element. A layer 8 of an organic photoemissive materialis provided over the anode electrodes 6 and a second array ofstrip-shape electrodes 10, which constitute the cathode elements for thedisplay element, are provided over the photoemissive layer 8. It can beseen from FIG. 1 that the respective arrays of anode and cathodestrip-shape electrodes 6, 10 are arranged substantially orthogonal toeach other. If a voltage is applied between any two of the strip-shapeelectrodes, a current will pass through that part of the photoemissivelayer 8 arranged in the area where the two electrodes overlap. Thematerial of the photoemissive layer behaves as a light emitting diode,and hence that part of the photoemissive layer in the overlap area ofthe two electrodes to which the voltage is applied will emit light. Thiscan be seen more clearly with reference to FIG. 2.

From FIG. 2 it can be seen that the pixels of the display are each madeup of an organic LED coupled between respective strip-shape anode andcathode electrodes. The strip-shape anode electrodes are, for example,decoupled from ground potential by a high impedance circuit, indicatedby a value Z in FIG. 2. Data signals, indicated by voltages V₁ to V₄ inFIG. 2, are applied to the cathode electrodes of the array. At the sametime, the strip-shape anode electrodes are selectively coupled directlyto ground potential. Hence, for the example shown in FIG. 2, when thevoltage V₁ is applied to the left most strip-shape cathode electrode,the organic LED L₁ will emit light. Likewise, when voltages V₂ to V₄ areapplied to the cathode electrodes 10, the LED's L₂ to L₄ willrespectively emit light.

Addressing schemes of the above type are called passive matrix schemesbecause there are no active elements located within the display area todrive the LED's to emit light. The light emission results purely fromthe data signals, in the form of voltage pulses, provided from the frameor boundary area of the display device to one of the sets of strip-shapeelectrodes, either the cathode or anode electrodes. However, the thinstrip-shape electrodes have electrical resistance, and this electricalresistance becomes larger with increase in the length of the strip-shapeelectrodes. Hence, if the size of the display area of the display deviceis made larger, the length of the strip-shape electrodes increases and,it follows, that the electrical resistance of the strip-shape electrodesis also increased.

The displays are driven from the side edge of the display and hence,when a voltage pulse is applied to any particular electrode, the voltageactually applied to the pixels underlying that electrode will decreasewith the distance of any pixel from the edge of the display to which thevoltage pulse is applied due to the electrical resistance of theelectrode. The potential drops along the electrodes can becomesignificant in comparison to the LED drive voltages. It will beappreciated, therefore, that if the electrodes are relatively long, thevoltage applied to a pixel located at the distal end of the electroderelative to the driven edge, will be significantly less than the voltageapplied to a pixel located close to the driven edge. The brightness ofthe display decreases therefore with increase in distance from thedriven edge and since the brightness-voltage characteristic of the LEDdevices is non-linear, this gives rise to non-uniform brightness ofdisplayed image.

Additionally, the intensity of the light emitted from an LED display isa function of the peak illumination intensity of the individual LEDdevices and the number of lines of pixels in the actual display area ofthe display. This is because the LED's of the display are addressed bypulse operation in a frame period. The time period during which any LEDmay be addressed is known as the duty ratio and is equal to t_(f)/N,where t_(f) is the frame period and N is the number of lines in thedisplay. It follows, therefore, that if the number of lines in thedisplay is increased, the duration for which any pixel may be addressedis decreased. The peak intensity of luminance from an LED occurs when itis addressed and this is averaged over the frame period. Therefore, toprovide a flicker-free display, as the size of the display area isincreased, and the number of lines in the display also increases tomaintain resolution, the peak intensity of light emitted from the LEDdevices must be compensated to maintain a required output intensity forthe display because it is only possible to address the LED devices for ashorter duration during a frame cycle. This can be particularlyproblematical for organic LED devices because of their very fast riseand decay times which means that they do not manifest an intrinsicmemory characteristic.

The peak intensity of the LED devices can be increased by increasing thevoltage of the pulses used to address the LED devices. It can beappreciated, therefore, that as display size, and hence the number ofscan rows, is increased, relatively high voltage pulses are required todrive the LED devices at a high current density and thereby providesufficient light output intensity from the display. This is aconsiderable disadvantage, as the long term reliability of the lightemitting devices can be impaired and when the display is incorporated ina device powered from an internal battery supply, such as a laptopcomputer, larger, heavier and more expensive batteries must be used.However, the use of such relatively high voltage pulses gives rise tofurther problems concerning operation of the LED's.

It is known that in LED devices, the possibility of recombination ofelectron hole pairs, which produces the light emission, can decreasewith an increase in voltage. This is because the optimum region foroperation of a LED device is what is commonly known as the‘recombination zone’.

The operational characteristic of a typical LED is shown in FIG. 3,which shows how luminance and device efficiency vary in relation to thecurrent and voltage applied to the device. It can be seen from FIG. 3that, once a current threshold is reached, as the current passingthrough the device is increased, the luminance of the device alsoincreases. However, with regard to efficiency, it can be seen thatdevice efficiency peaks very quickly once the device starts to emitlight. With further increase in voltage applied to the device, theefficiency falls quickly to a relatively low efficiency level, as shownin FIG. 3. For organic polymer LED's the peak efficiency occurstypically in the range of about 2.2V to about 5V, whereas, when theapplied voltage is in the range of about 10V to 20V, the efficiency ofthe device has fallen back to such a low level that it becomesinefficient and impractical to use such LED'S. Device efficiency is akey issue for many practical applications of LED displays as theequipment incorporating the display is frequently required to operatefrom an internal battery source.

This sharp decrease in device efficiency arises because, as the voltageapplied between the anode and cathode for a LED is increased, therecombination zone migrates towards one of the device electrodes.Because, the shape of the recombination zone depends on the appliedvoltage, with passive matrix addressed displays it becomes increasinglydifficult to provide sufficient display light output intensity becauserelatively high voltage pulses are required to drive the LED devices,which, in turn, means that the LED devices can no longer be operated inthe optimum recombination zone and, therefore, at an acceptable level ofefficiency.

To summarise therefore, display devices typically contain more than 200lines in order to provide sufficient resolution in the displayed image.Therefore, the LED's have a relatively low duty ratio which iscompensated by increasing the voltage applied to the LED's. However,this gives rise to lower operating efficiency of the LED's, which inturn decreases the luminance of the LED's, as shown by FIG. 5. These twooperational difficulties are inter-related and compound each other and,furthermore, they increase disproportionately with an increase in thenumber of lines in the display.

Active matrix addressing schemes are, therefore, frequently adopted forLED displays. An exemplary active matrix addressing scheme for anorganic polymer LED display device is shown in FIG. 4, which illustratesfour pixels of the display device. An active matrix driving schemeincludes arrays of row and column address lines shown as X₁ and X₂, Y₁and Y₂, in FIG. 4. These address lines are in the form of thinconductive strips along which pixel selection signals and data signalscan be fed to the pixels of the display device. Each pixel of thedisplay device is provided with two transistors, shown as T₁ and T₂ inFIG. 4. Further lines are also provided along which a supply voltageV_(ss) can be fed to the transistors at each pixel.

When it is desired to energise any particular pixel and so cause the LEDlocated at that pixel to emit light, a select voltage pulse is suppliedalong a row address line, for example, row address line X₁ in FIG. 4.This voltage pulse is received by the gate electrode G of transistor T₁causing transistor T₁ to switch ON for the duration of the voltagepulse. Assuming that the top left pixel is required to emit light, adata signal is applied to the source of transistor T₁ which is ON. Thedata signal, shown as Data 1, is passed by transistor T₁ to a capacitorcoupled to the gate electrode of transistor T₂. The data signal istherefore stored as a voltage in the capacitor.

Transistor T₁ operates as a switch, whereas transistor T₂ operates as acurrent driver for the organic LED, which is coupled to the supplyV_(ss) via the transistor T₂. When operating as a current driver, thecurrent at the drain of transistor T₂ will be a function of themagnitude of the voltage stored in the capacitor, which is proportionalto the data signal, Data 1. Hence, the current flowing through theorganic LED, which determines the illumination intensity of the LED, canbe controlled by variation of the signal Data 1.

The data signals are arranged so that the LED's always emit light duringoperation of the display and, therefore, lower operating voltages can beused. Hence, the use of the driver transistors at each pixel of thedisplay enables the LED's to be operated at lower operating voltages,and hence, much higher efficiency. FIG. 5 shows the typical operatingefficiencies of LED displays when operated by active and passive matrixaddressing schemes. The operating efficiency of the LED's is ofparamount importance and is the primary reason why active matrix schemesare frequently adopted for LED displays. Because the driver transistorsare located at each pixel of the display, they need to be fabricatedover a relatively large area and, hence, thin film transistors (TFTs)are used as the driver transistors in an active matrix addressingscheme. Hence, active matrix displays are commonly referred to as TFTdisplays.

The two most common types of TFTs are those where the layer ofsemiconductor material comprises either polysilicon or amorphoussilicon. More recently, TFTs have also been fabricated using organicmolecules or polymers as the semiconductor layer. However, because oftheir higher carrier mobility, polysilicon TFTs are usually used as thedriver transistors in active matrix displays for organic light-emittingdiode displays. With organic active matrix displays, as two drivertransistors need to be provided for each LED pixel of the display, thetransistor fabrication costs are relatively high because of thecomplexity of the fabrication techniques which must necessarily beadopted. In particular, when polysilicon driver transistors are used, ahigh temperature process must be used to provide the polysiliconsemiconductor layer. These increased costs, particularly when thedisplay area is made larger, negate the cost advantages provided by theorganic polymer materials. Non-uniformity of transistor performance isalso an issue. Again, this is particularly problematical for large areadisplays, because the large number of transistor drivers must befabricated over the larger area of the display, giving rise to increasedprocessing concerns and a reduction in the yield of fully functionaltransistor devices. For this reason ‘redundant’ driver transistors areusually provided, further increasing the cost of the display.

As mentioned above, polysilicon has, to date, been the preferredmaterial for TFT fabrication because of its relatively high mobility.Typically, polysilicon TFTs exhibit a mobility of between 100 to 500cm²/Vs, whereas amorphous silicon TFTs exhibit a typical mobility of 0.1to 1 cm²/Vs, and organic TFTs exhibit a mobility of 0.001 to 0.1 cm² Vs.Organic LEDs are current driven devices, so in the driver circuit shownin FIG. 4, it is important to maximise the drain current provided by thetransistor T₂.

The drain current Id of a TFT can be expressed as:

${Id}\;\alpha\frac{\mu\;{WC}}{L}$

Where μ is the mobility of the semiconductor

W is the width of the transistor channel region

L is the length of the transistor channel region

C is the capacitance of the gate

Therefore, the drain current Id is proportional to the mobility of thesemiconductor. Furthermore, the drain current is also proportional tothe channel width but inversely proportional to the channel length.Hence, if polysilicon TFTs are used for the drive circuits, therelatively high mobility enables the footprint of the transistorstructure within each pixel of the display to be minimised, which is animportant consideration for polysilicon and amorphous silicon type TFTs,as both devices are opaque. Because such TFTs are fabricated using hightemperature processes, they are usually formed on the rear surface ofthe screen (the substrate) of a display in advance. of the formation ofthe light-emitting elements and, hence, the footprint of the TFTs willnot transmit light emitted by the LEDs to a viewer of the display. Theproportion of the display which is able to pass the emitted light to aviewer is known as the aperture ratio, and for relatively small sizedisplays, such as those used in mobile telephones, an aperture ratio ofabout only 50% is achievable. That is, only about one-half of theavailable display area is able to display information to a viewer, withthe remaining one-half of the display area being occupied by the opaqueTFTs of the driver circuits and the conductor lines used to access thepixel located driver circuits. Even for large area displays it isdifficult to achieve an aperture ratio of greater than about 70 to 80%,so the reduction of illumination efficiency arising from the use ofopaque polysilicon or amorphous silicon TFTs arranged towards the frontviewing side of the display is significant, irrespective of the size ofthe display.

It is known that organic TFTs can be fabricated from organic moleculesor polymers which have a band gap providing transparency to radiation inthe visible spectrum. However, such transistors have relatively lowmobility and thus it has not been possible, to date, to use such organicTFTs for the active matrix drive circuit shown in FIG. 4. Displays havebeen demonstrated where an organic TFT has been used as the switchingtransistor T₁ but, to date, is has not been possible to use an organicTFT for the current driver transistor T₂ because the low mobility of thedevices means the device footprint must be made so large to providesufficient channel width to compensate for low mobility that thetransistors T₁ and T₂ cannot be accommodated within the area availablefor each pixel of the display. Hence, the advantage of usingsubstantially transparent TFTs for the active matrix driver circuits,which would enable aspect ratios approaching 100% to be realised, hasnot, thus far, been possible using the known arrangements for activematrix displays.

A further concern arises from the parasitic capacitance which existsbetween the driver lines to the driver transistors. In liquid crystaldisplays, the active liquid crystal material is located between theanode and cathode driver lines. The liquid crystal layer is usually inthe range of 2 to 10 microns in thickness and, therefore, the parasiticcapacitance arising between the driver line and counter common electrodeis relatively small. However, for organic LED displays, the organicmolecular or polymer layer is very thin, typically a few hundrednanometers in thickness. Hence, the parasitic capacitance is relativelylarge in comparison to LCD displays and this parasitic capacitancelimits the speed at which the displays may be operated, which becomesparticularly problematical as the display area increases. This isbecause it becomes necessary to address the display at a higher speed asthe size of the display becomes larger in order to maintain the qualityof the displayed image, but this gives rise to conflict because of thecapacitance of the electrodes. Additionally, as the display sizeincreases, the length, and hence the electrical resistance of the driverlines also increases, which again limits the speed at which the displaysmay be operated.

SUMMARY OF THE INVENTION

It can be appreciated that for large area displays, difficulties arisewhether active or passive driving schemes are adopted and thesedifficulties become more problematical when organic or polymer LEDs areused as the display light emitting elements. For very large areadisplays, such as for example those used to display images in publicplaces, it is known to combine a number of displays to provide the verylarge area display. However, each display making up part of such a verylarge area display is a separate display device. Whilst the use of alarge number of display devices can reduce the length of the driverlines in comparison to an equivalent size display made up of a singledisplay device, each display device of the large area displaynevertheless includes the relatively long address lines for addressingthe light emitting elements of the display. As such these displaydevices continue to suffer from the concerns outlined above. Therefore,there is a significant need to provide an improved type of displaydevice in which the above concerns are alleviated.

According to a first aspect of the present invention, there is provideda display device comprising a plurality of display segments arranged ona substrate, each display segment having a display area of pixelsdefined by an array of display elements, and a drive circuit for drivingthe display elements arranged within the display area.

In a preferred arrangement, the display elements comprise organicpolymer light emitting diodes.

The display elements may be arranged between a passive array of cathodeelectrodes and a passive array of anode electrodes, and the drivecircuit comprises a first drive circuit for providing signals to thearray of anode electrodes and a further drive circuit for providingsignals to the array of cathode electrodes.

Alternatively, each of the display elements may include a respectivedrive circuit comprising a thin film switching transistor and a thinfilm current driver transistor, thereby to provide active matrix displaysegments.

Preferably, the switching transistors and the current driver transistorscomprise organic or polymer transistors.

The organic molecules or polymer may be selected so that the transistorsare substantially transparent to visible light.

Advantageously, the current driver transistor comprises a source and adrain region each region formed as a plurality of longitudinallyextending sections joined at one end by a plurality of transverselyextending sections and wherein the longitudinally extending sections ofthe source region are interdigitated with and spaced from thelongitudinally extending sections of the drain region, thereby toprovide a spacing between the interdigitated longitudinally extendingsections of serpentine shape whereby the thin film transistor isprovided with a channel region having a channel length equal to thewidth of the spacing and a channel width extending the length of theserpentine shape spacing.

Each display segment may comprise gate lines and data lines foraffording control signals to the driver circuits, the gate lines anddata lines comprising a conductive organic or polymer material.

The gate lines and driver lines may each comprise a bi-layer structurehaving a first layer comprising the conductive organic or polymermaterial and a further layer comprising an inorganic conductivematerial.

In a further embodiment, each display segment comprises a cathodeelectrode and an anode electrode one of the electrodes being common toall pixels of the display segment and the other of the electrodes havingan electrode pattern so as to provide a respective electrode area foreach pixel of the display segment and wherein the driver circuit iscoupled to the anode and cathode electrodes and arranged on the oppositeside of the display elements to the substrate.

In a second aspect of the invention there is provided a method forfabricating a display device comprising providing a plurality of displaysegments on a substrate, each having a display area of pixels defined byan array of display elements and arranging a drive circuit for drivingthe display elements within the display area.

The active display elements may be fabricated from an organic or polymermaterial which, advantageously, is deposited by an inkjet print head.

According to a third aspect of the invention there is provided a thinfilm transistor comprising a substrate, conductive polymer source anddrain regions each formed as a plurality of longitudinally extendingsections joined at one end by a plurality of transversely extendingsections and wherein the longitudinally extending sections of the sourceregion are interdigitated with and spaced from the longitudinallyextending sections of the drain region thereby to provide a serpentineshaped spacing between the source and the drain regions and the thinfilm transistor with a channel region having a channel length equal tothe channel width of the spacing and a channel width extending thelength of the serpentine shape spacing.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way offurther example only, with reference to the accompanying drawings inwhich:

FIG. 1 is a schematic representation of a passive matrix addressingscheme for an LED display;

FIG. 2 is a schematic representation showing how the LED's of thedisplay shown in FIG. 1 may be addressed;

FIG. 3 is a plot showing variations of luminance and efficiency for atypical LED device;

FIG. 4 is a schematic representation of an active matrix addressingscheme for an LED display;

FIG. 5 is a plot showing the increase in efficiency which can beachieved by an active matrix addressing scheme in comparison to apassive matrix addressing scheme;

FIG. 6 is a schematic cross-sectional representation of a liquid crystaldisplay device;

FIG. 7 is a schematic representation of a display segment of a displaydevice according to the present invention;

FIG. 8 is a schematic representation of a prior art display device;

FIG. 9 shows a display device in accordance with the present inventionwith drive circuits mounted on the back of the display device;

FIG. 10 shows a partial cross-sectional view of a display segment havingdrive circuits located within wells located on a surface of a substrate;

FIG. 11 shows a partial cross-sectional view of a display segment havingdrive circuits located on the front of the display segment;

FIG. 12 is a schematic representation of an active matrix addressingscheme in accordance with the present invention;

FIG. 13 is a schematic representation showing how a source, gate anddrain may be configured for a driver transistor of the addressing schemeillustrated in FIG. 12;

FIG. 14 is a plot showing the absorption characteristic of an organicpolymer to ultraviolet and visible light;

FIG. 15 is a schematic cross-sectional view of a display pixel;

FIG. 16 is a schematic cross-sectional view of an active matrix displaydevice;

FIG. 17 is a schematic cross-section view of an organic polymertransistor;

FIG. 18 is a schematic cross-section view of a direct pixel driveaddressing scheme;

FIG. 19 is a schematic cross-section view of another embodiment of adirect pixel drive addressing scheme;

FIG. 20 is a schematic view of a mobile personal computer incorporatinga display device according to the present invention;

FIG. 21 is a schematic view of a mobile telephone incorporating adisplay device according to the present invention; and

FIG. 22 is a schematic view of a digital camera incorporating a displaydevice according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For a liquid crystal display, the liquid crystal material, being a fluidmaterial, must be contained between a substrate and a front plate orpanel for the display. Hence, the addressing lines and the drivertransistors (if an active matrix is used) are located between thesubstrate and front plate within the liquid crystal material itself, asshown in FIG. 6. With such a construction, the liquid crystal pixelsmust, in practice, be driven from the edges of the display, otherwise,it becomes necessary to gain access to the addressing lines by providingholes through the front and rear panels of the display, which is not apractical proposition.

It follows that, with a liquid crystal display, as the size of thedisplay area is increased, the length of the addressing lines will alsoincrease, and the displays will suffer from the addressing problemsdescribed above, irrespective of whether an active or a passive matrixaddressing scheme is adopted.

In an organic or polymer LED, the active organic material may comprise apolymer material or an organic small molecule type material. For apolymer material, this is deposited in liquid form but once depositedonto the substrate and dry, is a solid and relatively flexible material.Small molecule type materials are deposited by evaporation but,likewise, these are also, once deposited, solid and relatively flexiblematerials. Hence, for both polymer and small molecule type materials,the active organic material does not need to be retained on thesubstrate by the provision of the front plate, even though such a plateis provided in the finished display device to provide physical andenvironmental protection for the light emitting devices. Therefore, withthe present invention it has been realised that the pixels which providethe picture forming elements of the display can be addressed from anyposition within the display, including from the rear of the display, andnot just from the edge of the display. Furthermore, it has also beenrealised that, as the picture forming elements can be addressed from anyposition, the picture forming area of the display device can besub-divided into many display segments, each provided with its ownaddressing scheme, giving rise to significant benefits in comparison toknown arrangements for addressing schemes.

Irrespective of display size and whether an active or passive matrixaddressing scheme is used, a significant improvement in displayperformance can be realised. If a passive matrix addressing scheme isused, the length of the driver lines to the display elements can besignificantly reduced in comparison to a conventional display device asthe driver lines need to extend only within a segment and not the entirelength or breadth of the display. This becomes particularly advantageousfor a large area display device as the large display area can beconstituted by a large number of small display segments, each havingrelatively short length addressing lines driven and extending onlywithin each segment. This provides reduced resistance for the driverlines and the display intensity can, therefore, be improved because, forany given size of display, lower voltage driving may be used enablingthe LED devices to be operated in their optimum recombination zone, evenfor a large area display. Additionally, the addressing speed of thedisplay can also be increased as there is reduced parasitic capacitancebetween the arrays of co-operating anode and cathode electrodes composedof the relatively short addressing lines. As will be appreciated fromthe above description, the division of the display into a number ofsmall segments with drive circuits arranged within the segments providesa display with improved contrast and resolution of displayed image.

Moreover, with a conventional passive matrix addressing scheme with thedisplay driven from a side edge, only one line of the display emitslight at any one time because the lines are addressed sequentiallyduring a frame period. In comparison, an active matrix addressing schemeis beneficial because all of the pixels are arranged to emit light atall times. However, with the driver scheme of the present invention withthe driver circuits arranged in the display segments of the displayarea, more than one passive matrix segment can be addressed at any pointin time. Hence, the segments can be arranged to emit light in a mannermore akin to an active matrix addressing scheme, thereby increasing theluminance from the display.

Additionally, very large area displays can be achieved by combining alarge number of display segments onto a common single substrate and asthe organic or polymer material is relative flexible, such large areadisplays can be fabricated using a continuous batch process in which aweb of spoolable plastics material is fed past various processingstations to achieve display fabrication. Therefore, the cost benefits oforganic polymer displays can be more easily realised by adopting anumber of display segments, each with its own addressing scheme andassociated drive circuitry.

Significant benefits also arise when the present invention is used withactive matrix type displays. With active matrix type displays it isstill necessary to scan each line of pixels in the display at anappropriate scanning frequency. As the size of display is increased andhence the number of lines to be scanned is also increased, the scanningfrequency also needs to increase to maintain the quality of displayedimage. By sub-dividing the display into a number of segments, the numberof lines of pixels to be scanned is effectively reduced and hence thescanning frequency can also be reduced. The active matrix addressingscheme shown in FIG. 4 contains gate lines X₁ and X₂ and data lines Data1 and Data 2. By sub-dividing the display into display segments both thegate lines and the data lines are effectively reduced in length.Reductions in the lengths of the gate lines and the data lines are bothbeneficial because the lines exhibit capacitance and resistance whichlimit the speed at which scanning of the display pixels can be carriedout. The reduction in length of the gate lines and the data lines alsoenables these lines to be fabricated from alternative materials, such asconductive polymers or conductive pastes containing carbons. Thesealternative materials provide additional benefits because the lines maybe fabricated using printing techniques, such as the use of ink jetprinters, without the need for expensive photolithographic or maskingsteps. Reduction in the scanning frequency by applying sub-divideddisplay is particularly important when organic or polymer TFTs are usedas the switching transistors T₁, because the time required to charge thecapacitance tends to be long due to their low mobility.

Division of the display into a number of display segments also enablesthe use of an alternative type of addressing scheme which, in thecontext of the present invention, will be referred to as a “Direct PixelDrive” scheme. With Direct Pixel Drive, the pixels in any of the displaysegments each can be driven directly by a drive circuit IC arrangedwithin the display area of each segment. Direct Pixel Drive may beregarded as an alternative driving scheme lying between the conventionalpassive matrix scheme with overlapping arrays of anode and cathodeelectrodes, as shown in FIG. 7, and the conventional active matrixscheme with a switching and a driver transistor physically located ateach pixel, as shown in FIG. 4.

Referring to FIG. 7, a display segment 2 for a passive matrix displaydevice comprises a substrate 4 supporting an array of anode electrodes6. A layer 8 of electroluminescent organic or polymer material isprovided over the anode electrodes 6. An array of cathode electrodes 10is provided over the layer 8. The electroluminescent polymer materialmay comprise a conjugated polymer including, preferably, a fluorenegroup. When a voltage is applied between the anode elements and thecathode elements, such as between anode element 6 a and cathode element10 a, a current flows through the polymer material located in that partof the layer 8 where the elements 6 a and 10 a overlap, shown as shadedarea 12 in FIG. 7. This causes the shaded area 12 to emit visible lightand thus provide, a light emitting display element for the display. Theshaded area 12 constitutes, therefore, in combination with theoverlapping parts of the elements 6 a and 10 a, one of the pixels forthe display segment.

The display segment 2 also includes a first drive circuit 14 forproviding drive signals to the array of anode electrodes 6 throughconductive tracks 16 and a further drive circuit 18 for providing drivesignals to the array of cathode electrodes 10 via conductive tracks 20.The pixels of the segment 2 shown in FIG. 7 in combination provide adisplay area 22 for the segment and this is delineated in FIG. 7 by thebold dotted rectangle. It can be seen from FIG. 7 that the drivecircuits 14 and 18 are arranged within this display area of the displaysegment. Thus, the physical lengths of the arrays of anode and cathodeelectrodes 6, 10, which form a passive matrix addressing system for thesegment 2, can be kept to a minimum as they are addressed by drivecircuits from within the segment itself.

It follows, therefore, that the electrical resistance of the—electrodesas well as any parasitic capacitance which may be created between theanode and cathode electrodes, are significantly reduced in comparison tothe anode and cathode electrodes of a conventionally addressed displaydevice in which the equivalents of electrodes 6 and 10 would need toextend the entire length and breadth of the device. As describedpreviously, this enables lower voltage drive signals to be adopted andthe display elements, which constitute organic light emitting diodes, tobe operated within their optimum recombination region, providing adisplay with much higher efficiency than known display devices.

FIG. 8 illustrates schematically how a prior art known passive matrixdisplay device 24 may typically be configured. To ease explanation likereference numerals have been used in FIGS. 7 and 8 to signify theequivalent features of both devices. The display device 24 shown in FIG.8 is not configured as a plurality or number of display segments. Thedisplay area 22 is, therefore, constituted by a single unitary displayhaving anode and cathode electrodes 6, 10 extending the entire lengthand breadth of the display area of the device. As such, the anode andcathode electrodes of the display device 24 are of significantly longerlength than the anode and cathode electrodes of the display segment 2shown in FIG. 7, giving rise to the concerns as described previously.Furthermore, as will become apparent from the description following,because of their location within the active display area, the drivecircuits 14 and 18 of the display segment of FIG. 7 do not require to beencapsulated prior to mounting within the display area, as effectiveencapsulation can be carried out at a later stage as an integral step inthe fabrication of the display device. In essence, therefore, the drivecircuits of FIG. 7 may be unencapsulated integrated circuit devices ofvery small physical size.

In contrast, the drive circuits 14 and 18 of the display device of FIG.8 are necessarily provided as fully encapsulated integrated circuits asthey are located outside of the display area and wired to the respectiveanode and cathode electrode arrays through respective edges of thedisplay. Hence, the drive circuits 14 and 18 of FIG. 8 are much largerin physical size and significantly more expensive than the equivalentcircuits of the display segment shown in FIG. 7, because of theencapsulation and pin configuration necessary in such devices to allowfor the external wiring to the anode and cathode arrays. Therefore, itwill be appreciated that by interconnecting a number of display segments2, not only can the total display area be made larger for a given sizeof display device housing, because the drive circuits 14 and 18 arecontained within and not at the sides of the display area, but also alower cost, thinner or less bulky display device may be achieved as thedrive circuits shown in FIG. 7 may be provided as bare unencapsulateddevices which are subsequently encapsulated by the provision of a thinencapsulation layer.

FIG. 9 shows a display device 26 having a number of interconnecteddisplay segments 2. As with FIGS. 7 and 8, like reference numerals havebeen used to indicate like features of the display. It should be notedthat FIG. 9 shows the back view of the display, so in this embodiment,the drive circuits 14 and 18 of each display segment 2 are arrangedbetween the array of anode elements 6 and the substrate 4 which, toassist clarity, is not shown in FIG. 9.

In the display device 26, the respective drive circuits 14 for the anodeelectrodes 6 of display segments 2 are interconnected by interconnects28, and the cathode electrode drive circuits 18 are interconnected byinterconnects 30. These interconnects serve, therefore, to combine thedisplay segments 2 into the unitary display device 26. As can be seenfrom FIG. 9, the anode and cathode electrodes 6, 10 are maintained asrelatively short electrode strips, so although a large area display isprovided, this is achieved without the need for relatively long anodeand cathode electrodes, with their relatively high resistance andparasitic capacitance. In essence, this is made possible by effectivelysub-dividing the passive addressing matrix to provide display segments,and locating the drive circuits 14, 18 within the display area of eachsegment. Furthermore, because the drive circuits 14, 18 are providedwithin each segment, the display area of the display device can befurther increased to a very large size by including and interconnectingmore display elements. But, once again this is achieved without the needto increase the length of the relatively short anode and cathodeelectrodes 6,10.

It should be appreciated that in the display device of FIG. 9, the layer8 of emissive organic polymer material (which is not visible in FIG. 9)is disposed between the arrays of anode and cathode electrodes 6, 10. Assuch, the conductive tracks 20, which connect the drive circuits 18 onthe rear side of the display to the cathode electrodes 10 on the frontside of the display, must necessarily pass through the organic orpolymer layer 8. From FIG. 9 it can be seen that the conductive tracksare arranged such that they pass through the layer 8 in spaces 32between the anode electrodes 6 so as not to reduce the light emissivearea of the display device. Also, the organic polymer layer 8 is arelatively soft material, so it is relatively easy to provide suchconductive tracks from one side to the other of the layer 8 duringdevice fabrication. Alternatively, the conductive tracks may be providedby a series of pins, which may be provided on the drive circuit 18 andwhich pierce through the relatively soft and thin organic polymer layer.The same is also true for the display segment shown in FIG. 7, where theanode drive circuit 14 provided on the front side of the display must beconnected to the anode electrodes located under the organic polymerlayer.

FIG. 10 shows a further embodiment of the invention in which the drivecircuits 14, 18 are located in wells 34 provided in the substrate 4.Because the drive circuits can be provided as unencapsulated integratedcircuits the wells are of relatively small size and can be produced byany convenient process, such as by wet or dry etching, laser drilling,stamping or moulding of the substrate. The drive circuits can beinterconnected by suitably positioned interconnects, similar to thoseshown in FIG. 9.

FIG. 11 shows a partial section through the display segment 2 shown inFIG. 7 with the drive circuits 14, 18 mounted on the front side. Asmentioned previously, because the drive circuits are provided withineach display segment with its own passive array of anode and cathodeelectrodes, unencapsulated integrated circuit devices can be used forthe drive circuits 14, 18, with subsequent encapsulation by a thinencapsulation layer. Such a layer 36 is shown in FIG. 10. It will beappreciated that the encapsulation layer 36 can be extremely thin incomparison to the encapsulation packages used for stand alone integratedcircuits as typically used for drive circuits in prior art displays and,hence, a very thin display can be achieved by adopting the presentinvention.

Usually, organic polymer LED displays are fabricated with a transparentanode and an opaque cathode, such that light emission occurs through atransparent substrate, such as glass. A display device incorporating adriver circuit configuration in accordance with the present inventionmay be fabricated in any configuration with regard to the transparencyor opacity of the anode and cathode. For example, in the configurationshown in FIG. 10, with the driver circuits 14 and 18 arranged in wellsin the substrate, the anode may be made opaque and the cathode may bemade transparent, in which case light emission occurs through theencapsulation layer 36. In this instance the transparent cathode maycomprise, for example, a thin layer of Calcium (Ca) or Lithium Fluoride(LiF) with Indium Tin Oxide (ITO) and the opaque anode may comprise gold(Au) or platinum (Pt). Alternatively, in the configuration as shown inFIG. 11, with the driver circuits 14 and 18 arranged in overlyingrelationship with the cathode, the anode may be made transparent and thecathode opaque, in which case, light emission occurs through thesubstrate, which can also be arranged to be a transparent material. Inthis case, the transparent anode may comprise, for example, Indium TinOxide or Zinc Oxide (Z_(n)O₂) and the opaque cathode may comprise abilayer of Calcium/Aluminium (Ca/Al) or Lithium Fluoride/Aluminium(LiF/Al).

Alternatively, the transparent electrode array may be fabricated from anorganic polymer, such as PEDOT or polyanilene, or a bilayer structuresuch as a track of ITO and an overlying track of a conductive polymer.In the latter case, the use of the overlying organic polymer can assistin the planarisation of the ITO.

The layer 8 can be deposited by any suitable process but, being anorganic polymer material, it may conveniently be deposited by ejectingthe polymer solution from an inkjet print head. Alternatively, theorganic polymer may be deposited by spin coating. If the light emittinglayer comprise a small molecule material, this may be deposited byevaporation.

The present invention has been described in FIGS. 7 to 11 with referenceto a passive matrix type display but, as described above, thesub-division of the display into a number of display segments alsoprovides benefits when used with an active matrix type display.

It is mentioned above that one of the major issues with active matrixtype displays is the need to provide sufficient drive current from thecurrent driving transistor T₂ in the active matrix addressing schemeshown in FIG. 4 and that, to date, this has restricted the choice ofavailable materials to polysilicon in view of its high mobility incomparison to amorphous silicon or organic or polymer materials. As alsodescribed above, the polysilicon materials are opaque so the TFTsprovided for each pixel reduce the aperture ratio of the display.However, because the scanning frequency to program the voltage of thecapacitance can be reduced with the present invention, it is alsopossible to use transparent organic polymer TFTs for both the switchingand current driver transistors of the active matrix display.

The sub-divided display of the present invention also allows a lowerturn-on/off ratio in the switching transistors due to the smaller numberof gate lines in each display segment. The turn-on/off ratio of organicor polymer TFTs is generally lower (10³˜10⁶) than that of polysiliconTFTs (10⁷˜10⁸). The charge in the capacitor can escape during unselectedperiod (the off-state of the switching transistor), and that takes placemore significantly when TFTs, with a low turn-on/off ratio, are used.This limits the number of gate lines or a duty ratio of the signal atthe gate lines. Even with such TFTs, high resolution displays can beachieved with the sub-divided display of the present invention, becausethe number of the gate lines can be reduced by the segmentation.

FIG. 12 illustrates schematically an example of an active matrixaddressing scheme in a display pixel according to the invention. Theaddressing scheme comprises a driver circuit including a thin filmswitching transistor 102 and a thin film current driver transistor 104,similar to the scheme shown in FIG. 4. The switching TFT 102 functionsonly to pass a data signal on data line 106 when enabled by a voltagesupplied on a gate line 108. Because the switching TFT 102 functionsmerely as a pass gate, it does not have to provide a high drain current.Hence, the TFT 102 can be made of a relatively small size so that itoccupies only a small proportion of an area of a pixel. Furthermore,because the TFT 102 is not required to provide a high drain current itcan be fabricated from an organic or polymer material which may printedonto a supporting substrate, such as by ink jet printing.

The current driver transistor 104 is also advantageously fabricated froman organic or polymer material and this is made possible because thegate and driver lines have a relatively short length arising from thedivision of the display into an array of display segments. The currentdriver transistor at each pixel is required to maximise the draincurrent provided to the pixel LED from the supply V_(ss), which iscoupled to a source terminal of the TFT 104 via a comb-like electrode10, as shown in FIG. 12. Because the current driver TFT is fabricatedfrom an organic or polymer material it exhibits a relatively lowmobility and hence, to compensate, the width of the channel region mustbe made as large as possible, and the length of the channel region mustbe made as short as possible to provide sufficient drain current togenerate an appropriate level of light emission from the LED. The sourceand drain regions for the current driver TFT 104 are thereforefabricated as shown in FIG. 13.

The source region S and the drain region D are fabricated as comb-likeinter-digitated regions, as shown in FIG. 13. Because the source anddrain regions S, D are fabricated from a conductive polymer, theinter-digitated regions can be printed using an ink jet printing methodso as to easily and reliably provide a relatively small spacing, as lowas 2 to 30 microns using currently known techniques, between the sourceregion S and the drain region D. In essence, this spacing represents thechannel length of the TFT, shown as L in FIG. 13.

The channel width is provided by the length of the spacing between theinter-digitated fingers of the source region S and the inter-digitatedfingers of the drain region D. This is shown as W in FIG. 13 and extendsthe entire length of the spacing between the respective fingers of thesource region S and drain region D. By fabricating the driver transistoras shown in FIG. 13, the transistor can be provided with an extremelylong channel width W, which, in essence, is limited by the size of thedisplay element but may be provided to be in excess of 1000 micronswhilst continuing to achieve good image resolution.

The drain current of a TFT is proportional to the channel width W andinversely proportional to the channel length L. Hence, if the channelwidth W is made very large and the channel length L is made very small,such as those provided by the structure shown in FIG. 13, it is possiblefor a relatively low mobility organic/polymer transistor to providesufficiently high drain current to drive the pixel LED from the supplyV_(ss) if such a relatively large current driving transistor is allowedin each pixel.

FIG. 14 shows a plot of absorption against wavelength for an organicpolymer material and it can be seen that the material is essentiallytransparent to radiation having a wavelength greater than 410nanometers, i.e. visible light. Therefore, if the organic material forthe switching and current driver TFTs shown in FIGS. 12 and 13 isappropriately selected, the transistors of the active matrix addressingscheme can be made substantially transparent to visible radiation andcan therefore occupy the maximum space available in the pixel of thedisplay without reducing the aperture ratio of the display. The use ofan organic material having a band gap of more than three electron voltscan provide this transparency in the visible light spectrum.

In such a configuration, the large area of the gate electrode 104 actsas a part of a capacitor which holds the programmed potential. Due tosuch large area and the small distance between the gate electrode 104and the source 110 or the electrodes of the organic LED, the capacitanceof the capacitor tends to become large. When the switching transistor isan organic/polymer transistor, it takes considerable time to charge thecapacitance up to a certain voltage, leading to a limitation of thescanning frequency. The reduction in the scanning frequency achievablewith the segmented display of the present invention is also an importantaspect when organic molecules or polymer are used as the channelmaterial of the switching transistor 102.

The provision of a display as an array of display segments can providetherefore an active matrix addressing scheme having gate and data linesof reduced length in comparison to the length of the gate and data linesof an equivalent size display in the form of a single display device andthis reduced length of the gate and data lines enables the scanningfrequency for the display to be reduced, which in turn enablessufficient drive current for the current driven LEDs to be provided byorganic/polymer type TFTs. Because organic polymer TFTs can be used, theorganic material may be selected to be transparent to visible light andhence the TFTs may occupy substantially the whole of the surface area ofa pixel of the display without degradation of the aperture ratio.Furthermore, because an organic/polymer may be used as the channelmaterial of TFTs, the TFTs can be printed using a relativelystraightforward low temperature process, such as ink jet printing,micro-contact printing, screen printing or photo-patterning, without theneed for photolithographic or masking techniques.

FIG. 15 shows a schematic cross-sectional view through a pixel of thedisplay in accordance with this aspect of the present invention, withthe switching and current driver TFTs of the active matrix addressingscheme formed on a supporting substrate such as glass or plastic, withthe light emitter region formed on the TFTs and a common cathode formedover the light emitter regions of the display pixels. Because theswitching and driver transistor can be made transparent they can occupysubstantially the whole of the surface area of the pixel and the lightemitter region is able to emit light through the transparent transistorsand the transparent substrate. Because the gate, source and drainelectrodes and data lines to the active matrix driver circuit can alsocomprise transparent conducting polymers, the display can be fabricatedentirely using a low temperature printing process and an aspect ratio ofsubstantially 100% can be achieved, even when using an active matrixaddressing scheme. Therefore, a relatively low cost but high efficiencydisplay can be provided.

The display can also be fabricated by providing a patterned cathode incommunication with the active matrix TFTs, the patterned cathode beingarranged on a common light emitting layer overlying a common anode, asshown in FIG. 16. Again, the active matrix TFTs can be fabricated by anyof the printing or patterning processes referred to above so as tooccupy substantially the whole of the area of each pixel. Thisconfiguration is possible only when organic/polymer TFTs are used fordriving the light-emitting pixels, since organic/polymer TFTs can befabricated with low temperature processes which do not degrade theorganic/polymer light-emitting layer. When polysilicon oramorphous-silicon TFTs are fabricated, not only high temperatureprocesses but also high energy processes, such as plasma deposition andetching, ion implantation, W light exposure in lithography, arerequired. These processes destroy or degrade the organic/polymerlight-emitting layer, so such TFTs are necessarily fabricated on thesubstrate in advance of the fabrication of the organic/polymerlight-emitting layer. With the present invention, organic/polymer TFTscan be used as the transistors to drive the light-emitting pixels, andthis can completely change the conventional manufacturing process of theactive-matrix displays: the TFTs can be fabricated after theorganic/polymer light-emitting devices are formed.

In the structure shown in FIG. 16, the switching and driver transistorsare isolated from the patterned cathode by an insulator layer, which maybe an insulating polymer. As such, the organic light emitting layer andthe insulating layer are both relatively soft and hence it is relativelyeasy to provide a conductive path through to the buried common anode.

In the structure shown in FIG. 16, the TFTs are fabricated on thelight-emitting devices which have the common anode and the patternedcathodes. However, TFTs can also be fabricated on light-emitting deviceswhich have patterned anodes and a common cathode. In this case, holes inthe common cathode are required to obtain conductive paths to the commonanode from the top positioned TFTs.

Another important advantage of the top-TFT-structure is that thematerials of the electrodes, semiconductor and insulator of the TFTs arenot required to be transparent. For example, metal or metal colloid canbe used as the electrodes, and higher mobility material which is notcompletely transparent in visible range, such as polythiophene,poly(alkylthiophene), pentacene, copolymer of fluorene and bithiophene,polythienylenevinylene, thiophene-based oligomers, or phthalocyanine, asthe semiconductor. It is also possible to use n-type organic TFTs withsemiconductor which has high electron mobility, such as Pc2Lu, Pc2Tm,C60/C70, TCNQ, PTCDI-Ph, TCNNQ, NTCDI, NTCDA, PTCDA, F16CuPc, NTCDI-C8F,DHF-6T, pentacene, or PTCDI-C8.

It is evident from the above description that by suitable selection ofthe organic or polymer material, the active matrix addressing schemetransistors can be made substantially transparent. It is also possibleto select the electrode materials to be transparent, such as by the useof ITO or PEDOT for the transistor source and drain electrodes or theuse of PEDOT or polyanilene for the gate electrodes.

The conductive polymer, such as PEDOT, may also be used in combinationwith an inorganic conductor, such as ITO, to form the source/drainelectrodes and this provides additional advantages in an active matrixscheme. ITO is frequently used to fabricate the anode for passive matrixaddressing schemes because it is substantially transparent but exhibitsrelatively high conductivity. However, ITO, when deposited, is known toexhibit relatively poor planarisation which increases the difficulty ofdepositing further layers, especially an insulating layer, onto the ITOdeposited layer. Some p-type organic semiconductors exhibit anionisation potential higher than 5.0 eV. When ITO is used as theelectrode for such an organic or polymer type transistor, the differencein the energy levels of the ITO and organic polymer make hole injectiondifficult, which increases the contact resistance at the interfacebetween the ITO electrode and the organic semiconductor.

Conductive polymers such as poly-3,4-ethlenedioxythiophene (PEDOT) orpolyanilene have an ionisation potential lying between that of ITO andthe organic semiconductor materials which have an ionisation potentialhigher than 5.0 eV. Hence, if source/drain electrodes are used which arein the form of bi-layer structures comprising a first layer of aninorganic conductor and a second layer comprising of a conductiveorganic polymer, and the organic semiconductor is arranged in contactwith the conductive organic polymer layer to provide a structure, asshown in FIG. 17, hole injection into the semiconductor layer is madeeasier. For example, assuming that the inorganic conductor layer 120 isITO, which has a Fermi level of between −4.0 eV to −4.5 eV, the organicconductor layer 122 is PEDOT which has a Fermi level of about −4.6 eV to−4.8 eV, and the organic p-type semiconductor layer 124 has the energylevel of highest occupied molecular orbital of abut −5.0 eV. Because theenergy gap between the ITO layer 120 and the PEDOT layer 122 isrelatively small, the injection of holes from the ITO layer into thePEDOT layer is easier to achieve in comparison to hole injectiondirectly from ITO into the organic p-type semiconductor. Similarly,because the difference in the energy bands between the organic conductorlayer 122 and the p-type organic semiconductor layer 124 is alsorelatively small, the injection of holes from the organic conductorlayer to the organic semiconductor layer is also easier to achieve. Theconductivity of ITO is higher than most of the conductive polymers,leading to the high conductivity of the bi-layer electrode structurecompared to that of polymeric single layer electrodes. Hence, theoperational efficiency of the organic polymer transistor from a supplysource of defined voltage can be improved by the use of such aninorganic/organic bi-layer electrode structure. Such bi-layer electrodescan be fabricated by etching the inorganic layer by using the patternedpolymer layer as an etching mask. For example, patterned PEDOTelectrodes may be deposited onto a continuous ITO layer by ink jetprinting, micro-contact printing or screen printing, and this isfollowed by etching of ITO using an acidic etchant. Only the areas notcovered with the PEDOT electrodes are etched by the etchant, resultingin the inorganic/organic bi-layer electrode structure.

This improvement in operational efficiency is an important considerationfor the use of organic polymer transistors as the current drivertransistors of an active matrix addressing scheme because of therelatively low inherent mobility of such semiconductor devices.

The sub-division of a display into an array of display segmentsprovides, therefore, for a display with an active matrix-type addressingscheme, gate and data lines of reduced length, which in turn enables theuse of organic polymer-type transistors to be adopted for both theswitching and current driving transistors of the active matrix scheme.The organic polymer transistors may be fabricated with a low temperatureprocess without the use of photolithographic or masking steps, such asby ink jet printing, which enables the active matrix transistors to befabricated over the organic light emitting diodes of an organicelectroluminescent display. The organic polymer materials for the activematrix transistors can be selected to be substantially transparent tovisible radiation, thereby providing an improved aperture ratio for thedisplay. Hence, a relatively efficient and relatively high qualitydisplay may be fabricated at relatively low cost by using well provenprinting techniques. Due to this low temperature and low energy processfor fabricating TFTs, the TFTs can be fabricated after the preparationprocesses of the organic/polymer light-emitting diodes.

The sub-division of the display into a number of display segments andthe driving of the display pixels from a driver circuit within thedisplay area also enables a direct pixel drive addressing scheme to beused. Examples of such a scheme are shown in FIGS. 18 and 19, where likereference numerals have been used to indicate like parts of thestructures.

FIG. 18 shows a direct pixel drive addressing scheme using an array ofdiscrete anodes 200 arranged on a transparent substrate 202. The anodepattern is arranged so that an electrode area is provided for each ofthe pixels of the display segment. An organic polymer light emittinglayer 204 is provided over the array of discrete anodes and a commoncathode 206 is provided over the light emitting layer 204. A driver IC208 is provided to drive the pixel of the display and this is coupled toboth the common cathode and the discrete anodes by electrodes 210.

The array of discrete anodes 200 are fabricated from a transparentmaterial such as ITO or PEDOT so that the display may be viewed throughthe substrate. The common cathode 206, being arranged over the organicpolymer light emitting layer is preferably fabricated from a conductingpolymer which may be deposited by spin coating with subsequent etchingof the apertures for enabling the electrodes 210 to pass through thelight emitting layer to the discrete anodes.

The structure shown in FIG. 19 is, in essence, similar to the structureshown in FIG. 18 except that a common anode 212 and an array of discretecathodes are used for the display segments.

With the structures shown in FIGS. 18 and 19, a number of the displaysegments can be arranged in juxtaposition to provide a display deviceand as each display segment can be made relatively small, each can bedriven a respective driver circuit, such as the driver ICs shown inFIGS. 18 and 19 arranged on the rear of the structure, and the displaycan be driven without the need to scan the individual rows or columns ofpixels, as is the case with the generally used active or passive matrixaddressing schemes.

The display device of the present invention may be incorporated in manytypes of equipment such as mobile displays e.g. mobile phones, laptoppersonal computers, DVD players, cameras, field equipment; portabledisplays such as desktop computers, CCTV or photo albums; instrumentpanels such as vehicle or aircraft instrument panels; or industrialdisplays such as control room equipment displays.

Various electronic apparatuses using the above organicelectroluminescent display device will now be described.

<1: Mobile Computer>

An example in which the display device according to one of the aboveembodiments is applied to a mobile personal computer will now bedescribed.

FIG. 20 is an isometric view illustrating the configuration of thispersonal computer. In the drawing, the personal computer 1100 isprovided with a body 1104 including a keyboard 1102 and a display unit1106. The display unit 1106 is implemented using a display panelfabricated according to the present invention, as described above.

<2: Portable Phone>

Next, an example in which the display device is applied to a displaysection of a portable phone will be described. FIG. 21 is an isometricview illustrating the configuration of the portable phone. In thedrawing, the portable phone 1200 is provided with a plurality ofoperation keys 1202, an earpiece 1204, a mouthpiece 1206, and a displaypanel 100. This display panel 100 is implemented using a display deviceaccording to the present invention, as described above.

<3: Digital Still Camera>

Next, a digital still camera using an OEL display device as a finderwill be described. FIG. 22 is an isometric view illustrating theconfiguration of the digital still camera and the connection to externaldevices in brief.

Typical cameras use sensitized films having light sensitive coatings andrecord optical images of objects by causing a chemical change in thelight sensitive coatings, whereas the digital still camera 1300generates imaging signals from the optical image of an object byphotoelectric conversion using, for example, a charge coupled device(CCD). The digital still camera 1300 is provided with an OEL element 100at the back face of a case 1302 to perform display based on the imagingsignals from the CCD. Thus, the display panel 100 functions as a finderfor displaying the object. A photo acceptance unit 1304 includingoptical lenses and the CCD is provided at the front side (behind in thedrawing) of the case 1302.

When a cameraman determines the object image displayed in the OELelement panel 100 and releases the shutter, the image signals from theCCD are transmitted and stored to memories in a circuit board 1308. Inthe digital still camera 1300, video signal output terminals 1312 andinput/output terminals 1314 for data communication are provided on aside of the case 1302. As shown in the drawing, a television monitor1430 and a personal computer 1440 are connected to the video signalterminals 1312 and the input/output terminals 1314, respectively, ifnecessary. The imaging signals stored in the memories of the circuitboard 1308 are output to the television monitor 1430 and the personalcomputer 1440, by a given operation.

Examples of electronic apparatuses, other than the personal computershown in FIG. 20, the portable phone shown in FIG. 21, and the digitalstill camera shown in FIG. 22, include OEL element television sets,view-finder-type and monitoring-type video tape recorders, carnavigation systems, pagers, electronic notebooks, portable calculators,word processors, workstations, TV telephones, point-of-sales system(POS) terminals, and devices provided with touch panels. Of course, theabove OEL device can be applied to display sections of these electronicapparatuses.

Furthermore, the display devices of the present invention is suitablefor screen-type large area TV which is very thin, flexible and light. Itis possible to paste such large area TV on a wall, or to hang on a wall.The flexible TV can be rolled up when it is not used.

With the present invention, a large number of display segments can beinterconnected on a common substrate to form a large area display drivewithout the need to increase the voltage of the drive signals fed to theanode and cathode electrodes of the device or the need to increase thespeed at which the pixels of the display are scanned. Hence, althoughthe substrate 4 may be a rigid substrate, such as of glass, plastics orsilicon, the present invention lends itself very favourably to thefabrication of a display device on a spoolable plastics substrate,thereby facilitating the efficient fabrication of very large area, highspeed, high resolution displays with high efficiency.

The foregoing description has been given by way of example only and itwill be appreciated by a person skilled in the art that modificationscan be made without departing from the scope of the invention. Forexample, the invention has been described with reference to organicpolymer LED displays but may also be used with reflective type liquidcrystal displays. Furthermore, for passive matrix type displays, thedriver circuits for the anode and cathode have been described asseparate driver circuits. However, these driver circuits may beintegrated into a unitary driver circuit, in which case both the anodeand cathode may be driven by the unitary circuit. The present inventionhas been described with reference to an organic polymer material for useas the light emitting elements. However, small molecule materials mayalso be used to equal effect.

What is claimed is:
 1. A display device comprising: a first electrode; aplurality of second electrodes; a display element positioned between thefirst electrode and at least one of the plurality of second electrodes;a substrate positioned such that the at least one of the plurality ofsecond electrodes is positioned between the display element and thesubstrate; a first drive circuit overlapping with the display element,the first drive circuit being configured to control the first electrode;and a second drive circuit overlapping with the display element, thesecond drive circuit being configured to control the plurality of secondelectrodes.
 2. The display device according to claim 1, the first drivecircuit being positioned between the substrate and the at least one ofthe plurality of second electrodes, the second drive circuit beingpositioned between the substrate and the at least one of the pluralityof the second electrodes.
 3. The display device according to claim 1,the display element including emissive organic material.
 4. The displaydevice according to claim 1, the first electrode being a commonelectrode.
 5. The display device according to claim 1, the substrateincluding a plurality of transistors.
 6. An electronic apparatuscomprising: the display device according to claim
 1. 7. A displaydevice, comprising: a plurality of first electrodes; a plurality ofsecond electrodes; a display element positioned between at least one ofthe plurality of first electrodes and at least one of the plurality ofsecond electrodes; a substrate positioned such that the at least some ofthe plurality of second electrodes is positioned between the displayelement and the substrate; a first drive circuit overlapping with thedisplay element, the first drive circuit being configured to control theplurality of first electrodes; and a second drive circuit overlappingwith the display element, the second drive circuit being configured tocontrol the plurality of second electrodes.
 8. The display deviceaccording to claim 7, the first drive circuit being positioned betweenthe substrate and the at least one of the plurality of secondelectrodes, the second drive circuit being positioned between thesubstrate and the at least one of the plurality of second electrodes. 9.The display device according to claim 7, the display element includingemissive organic material.
 10. An electronic apparatus comprising: thedisplay device according to claim 7.